Radio receiver noise suppression circuit



Dec. 7, 1948. R. 1-. THOMPSON RADIO RECEIVER NOISE SUFPRESSION CIRCUITFiled Jan. 15 1944 INVENTOR ROBERT T. THOMPSON Patented Dec. 7, 1948NiTED STATES PATENT OFFICE.

RADIO RECEIVER NOISE SUPPRESSION CIRCUIT Robert T. Thompson, Oak Park,Ill., assignor to Zenith Radio Corporation, a corporation of IllinoisThis invention relates to high frequency wave translating apparatus, andmore particularly to such apparatus arranged to be responsive tofrequency deviation.

Many types of apparatus, such, for example, as frequency modulationreceivers, incorporate a device responsive to frequency deviation of amodulated carrier wave, and it is desirable that such devices be assensitive as possible to such requency deviation. That is, it isdesirable that the frequency deviation responsive circuit produce aslarge a voltage as possible upon deviation of the frequency of theincoming wave from a predetermined frequency. Various arrangements havebeen utilized for producing such a large voltage upon frequencydeviation. Such arrangements commonly incorporate a network whosereactance changes linearly very rapidly over a relatively narrowfrequency range, whereby, for a constant input voltage, the currentwhich flows through the network changes very rapidly as the frequency ofthe incoming wave changes over that relatively small frequency range.

It is generally desirable that such frequency deviation responsivedevices operate normally about a mean frequency in the relatively smallfrequency range over which they are linearly responsive. This isparticularly true in receivers arranged to receive and to demodulatewaves whose frequencies are modulated in accordance with signals. Suchfrequency modulation receivers are usually of the superheterodyne typeand incorporate carrier wave voltage limiters which impress on the inputof a frequency variation responsive device a voltage of fixed maximumamplitude whose frequency is modulated in accordance with a signal. Theloudness of the reproduced signal does not change substantially as themean frequency of the frequency modulated carrier wave is shiftedthrough the relatively small linear response band of the frequencyresponsive network unless the mean frequency of the wave approaches thelimits of linearity of the relatively narrow frequency band. In suchcase, there is not merely a reduction in the apparent loudness of thereceived signal, but there is considerable distortion. This distortionis accentuated as the mean frequency of the received or transformedcarrier wave assumes magnitudes further and further removed from amagnitude within the linear frequency response band of the frequencydeviation responsive circuit. Regions, defined by the so-called backslope of such frequency deviation devices, just outside of the linearresponse "band of the circuit,

are effective to demodulate the carrier wave, but produce effects notlinearly proportional to the frequency of the signal and for that reasonit is very desirable that for best reception the mean frequency of thereceived or modified carrier wave be kept inside the linear band and notin the nonlinear regions corresponding to the back slope which ispresent on each side of the linear portion of the frequency responsecharacteristic curve of the frequency deviation responsive circuit.

It is a specific object of this invention to provide an improvedfrequency modulation receiver which does not reproduce signals exceptwhen it is tuned correctly for best reception; that is, to arrange sucha receiver so that it will not reproduce undesired fuzzy signals due todemodulation on the back slopes of the frequency response characteristiccurve of the receiver.

More broadly, it is an object of this invention to provide a new andimproved frequency deviation responsive arrangement of high sensitivityin which there is substantially no response except when the frequency ofa wave impressed on the frequency variation responsive arrangement lieswithin the linear portion of its frequency range.

It is another object of the present invention to provide a simplearrangement which suppresses the distorted responses produced whenoperating in the range of either one of the back slopes of a frequencydiscriminator so that tuning of a frequency modulation receiverincorporating the discriminator is easier.

A corollary object of the present invention is to provide an improvedfrequency deviation responsive arrangement in which the frequencydeviation sensitivity is high when the frequency of a wave impressedthereon is within its linear range and which is low when the frequencyof such a wave is outside of its linear range.

It is frequently the practice in radio transmission to reduce the rangeof intensity variation of signals before modulating the carrier wave inaccordance with such signals, in order that sounds of the smallestintensity may be reproduced at the receiving station and in order thatsounds of the largest intensity may not produce frequency deviation tosuch an extent that the frequency of the carrier wave is increasedbeyond certain assigned limits.

It is another object of this invention to provide a frequency deviationresponsive arrangement which is substantially unresponsive to wavesoutside of its substantially linear response range and in which therange of intensity variation of demodulated signals may be adjusted asdesired.

Another object of this invention is to provide means responsive to anincorrect tuning condition of a frequency modulation receiver forsuppressing the unwanted response due to the back slope of thediscriminator characteristic.

Another object of this invention is to provide means responsive to acondition of correct tuning for enhancing the deviation sensitivity of areceiver for frequency modulated waves.

Another object of this invention is to provide a new and improvedlimiter in a receiver for frequency modulated signals in which thelimiter transmits signals only when the receiver is correctly tuned.

Still another object of the present invention is to provide improvedarrangements for controlling the deviation sensitivity of a frequencyresponsive network in accordance with the intensity of a signal passingthrough such network.

Yet another object of this invention is to provide improved arrangementsfor controlling the frequency deviation sensitivity of a frequencyresponsive system having a limiter circuit in which an electricalcharacteristic of such limiter circuit controls the deviationsensitivity.

Yet another object of this invention is toprovide an improved signalamplitude compression system incorporating frequency deivationresponsive apparatus in which voltages developed in either an amplifyingstage or limiter stage are utilized to control frequency deviationsensitivity.

Yet another object of this invention is to provide an improved signalamplitude expansion 5..

system incorporating frequency responsive apparatus in which voltagesdeveloped in either an amplifying stage or limiter stage are utilized tocontrol frequency deviation sensitivity.

The features of the present invention which are believed to be novel areset forth with particularity in the appended claims. This inventionitself, both as to its organization and manner of operation, togetherwith further objects and advantages thereof, may best be understood bywave of constant amplitude and output voltage response, plotted asordinate, in a frequency deviation response network on which suchcarrier wave is impressed. There is a linear portion of the line 5 inwhich the output voltagefor response, of the network changes far morerapidly than the frequency of the impressed carrier wave. There arenonlinear back slopes 6 and I at each end of the linear portion of curve5, over which output voltage changes nonlinearly as the frequency ofvoltage applied to such network changes.

One phase of the present invention relates to our arrangement forcausing a frequency deviation response network having the characteristicillustrated in Fig. 1 to produce an output voltage, or cause a response,substantially only when the frequency of a carrier wave impressed on thenetwork lies Within the limits of the linear part of curve 5. Such anarrangement is useful and highly desirable in a receiver for receivingfrequency modulated waves, and when incorporated in such a receiver itis practically impossible to mistune the receiver to the extent ofproducing demodulation on the back slopes 6 and 1.

The term receiver as used in this specification, is meant to include anydevice possessing at least a source of frequency modulated carrier wavesand a circuit for detecting the modulation signal on the carrier waves.Purely by way of example, a repeater in which the modulation signal isdetected and retransmitted without conversion to sound would fall in theclassification of a receiver, in the light of this specification.

Figure 2 is the circuit diagram of a particular type of frequencymodulation receiver in which frequency modulated waves intercepted byantenna H] are detected and the modulation signal is reproduced onspeaker Hi9. This receiver is of the superheterodyne type, includingvariably tuned circuits in the radio frequency amplifier stages 63 and73 and tuned circuits in a frequency converter or oscillator modulatorstage 14, the tuned circuits being adjustable to adjust the receiver toreceive frequency modulated Waves of any desired mean frequency.

Signals received on antenna in are applied to the primary 60 of antennatransformer 6| which has its output winding 62 tuned by condenser 62Aand has one terminal thereof connected to the control grid of variablemu discharge device 63, the other terminal of winding 62 being connectedeither to the cathode of device 63 through conventional bias resistance61 shunted by conventional by-pass condenser 68 or connected to thecathode of device 63 through serially connected condenser I20 andresistance 61, depending upon the position of switch I0.

Space current for device 63 is supplied from source 64 which has itspositive terminal con- 3 nected to the screen grid of device 63 and alsoto one terminal of tuned output circuit 65, 66 comprising the primarywindin 65 of output transformer H and variable tuning condenser 68, theother terminal of the tuned circuit 65, 66 being connected to the anodeof device 63. The negative terminal of source 64 is grounded andconnected to the cathode of device 63 through serially connectedresistance 61 so that space current flowing from the anode and screengrid of device 63 develops a control grid bias voltage across resistance61 for causing device 63 to operate over a suitable portion of itsoperating characteristic curve so as to produce suitable amplificationof signals applied across variably tuned circuit 62, 62A. Such signalsare amplified by device 63 and then appear across tuned output circuit55, 66 and then are transferred to the secondary winding 12 of outputtransformer H. Such amplified signals are again amplified in anotherradio frequency amplifier stage 13 and then applied to the input of theoscillator modulator stage 14.

The fixed frequency output voltage of the oscillator modulator orfrequency converter 14 is amplified in a fixed frequency intermediatefrequency amplifier l5 and is impressed across a tuned circuit includingthe primary winding 16 of the high frequency transformer 18, connectedin shunt with a tuning condenser TI. The tuned circuit comprisingelements 76, T! is adjusted to be resonant at the fixed mean frequencyat which the intermediate frequency amplifier I5 is operative.

The secondary winding 19 of the transformer 18 is connected in shuntwith a tuning condenser 59 and voltage developed thereacross isamplified, and limited in intensity on both half cycles f a v in aamplifyi g d miti di charge device 8|. The discharge device BI isassociated with a second discharge device 99 and with a circuitarrangement which causes the limiting and amplifying device 85 to passto its output circuit substantially no signal when the signal impressedon tuned circuit 19, Si! is below a predetermined intensity, and whichcauses the limiting and amplifying device 8! to pass to its outputcircuit a signal of substantially constant intensity when the signalimpressed on tuned circuit E9, 86 is above a predetermined intensity.

The output of discharge device Bl, thus modified, is impressed across. atuned circuit including. the primary 36 of a transformer, the primary 8h being connected in shunt with a tuning condenser 81 to form a tunedcircuit 85, 81 tuned to the same frequency as is the output circuit ofthe intermediate frequency amplifier 15. The tuned circuit 86, 81 isassociated with a frequency variation response network or frequencydiscriminator 97 of the well known resonant type, arranged to produce anoutput voltage whose intensity and polarity varies as the frequency of aconstant intensity wave impressed on the tuned circuit 86, 81 varies inamount and direction from a predetermined mean frequency.

The output voltage of the discriminator 9'! is applied between the gridand cathode of amplifying discharge device I 02, amplified thereby, anda first portion of such amplified voltage is applied through a couplingcondenser III to the plate of a rectifier such as a discharge deviceI31, rectified in discharge device 13! and then applied across theresistance 92 for controlling the conductivity of discharge device 90,and a second portion of such amplified voltage is applied to a secondamplifying circuit till and then applied to sound reproducer I09.

In particular, the audio voltage from discriminator 9'? is appliedacross the serially connected condenser l!) and resistance IN, theresistance EM being connected between the grid and cathode of amplifyingdevice I02 through conventional cathode biasing resistance Hi4 havingconnected in shunt thereto by-pass capacitance H35v for preventingdegeneration effects. Space current for discharge device I02 is suppliedfrom voltage source 66 which, has its positiveterminal connected to theanode of device Hi2 through serially connected resistance N13. Thecathode of device IE2 is grounded through cathode bias ing resistanceI04, and the resultingspace current fiowing through resistance I04produces a suitable negative control voltage for the control electrodeof device Hi2 suchthat. device, H12 op-., erates along a substantiallylinear portion of its control voltage versus space currentcharacteristic.

Signal voltages appearing across resistance 10.! are amplified by deviceI02 and, as mentioned previously, such amplified signals have a portionthereof applied to rectifier discharge device I31 for producing" acontrol voltage and another portion thereof which appears across outputcoupling resistance m3 applied to the terminals of the input circuitofamplifying circuit l0! through coupling condenser I08, condenser Hi8being connected to the junction point; of resistance I03 and anode ofdevice H32; and one terminal ofnetwork lll'l being grounded.

The limiting amplifier device 8| mentioned previously, is arranged tolimit the; maximum amplitude of both half cycles; of" an; impressedsignal Wave to less than a predetermined intensity, and to that end itscathode is grounded and its first or control electrode 106 is connectedto one terminal of the tuned circuit l9, 8!], the other terminal of thattuned circuit being connected to ground through a serially connectedresistance 82 connected in shunt with a by passing condenser 83 forby-passing high frequency currents around the resistance 32. The anodeH3 of device Si is connected to one terminal of the tuned circuit 863,81, the other terminal of that tuned circuit being connected throughserially connected resistances 8t and 85 to the positive terminal of avoltage source 6 of operating potential, the negative terminal of thesource 64 being grounded. A voltage dividing resistance 88 is connectedin shunt with the source 54, and its adjustable tap is connected to thesecond or screen electrode lid of device 8!, which screen electrodeconnected throu h a condenser 89 to the cathode of device 8|, thereactance of condenser 89 being sufliciently low at high frequencies toprevent substantial potential variation of electrode H 2.

Neglecting for a moment the fact that the discharge device 90 isconnected in circuit with discharge device SI, and assuming that thepotentials of the various electrodes of discharge device 81 are as theywould be adjusted by device 80 in the presence of a signal impressed onthe tuned circuit F9, 88 above a predetermined intensity such thatdevice 8! limits and transmits a signal wave to the tuned circuit 86,8?; limiting action in the discharge device 8| take place on both halfcycles of the wave in known fashion. Rectification of the wave on thecontrol electrode IE5 is efiective to develop a bias potential acrossresistance 82, whose polarity is as indicated, and, assuming theintensity of the impressed wave is sufficient, the control electrodeldli is driven far more negative during a part of each half cycle of theimpressed wave than the negative potential required to cut off anodecurrent in the device 8i. By this process negative half cycles of theimpressed wave are limited in intensity and appear on the anode I l3only with such limited intensity.

The potential of the screen electrode ll-l adjusted by the voltagedividing resistance 88 to a low value, such that positive half cycles ofthe impressed signal wave on control electrode H36 are by certain wellknown mechanisms also transmitted to the anode H3 in limited intensity.

In order to accomplish some of the purposes of this inventiomrthedischarge device is inter connected with the device 8! to cause thedevice 8i to cease transfer of the wave to the tuned circuit 86, 8! whenthe intensity of the wave on the tuned circuit 79', 80 drops below apredetermined intensity. The control electrode H22 of device 99 isconnected to that terminal of tuned circuit it, 80 which is connected toresistance The anode 523 of discharge device Bil is connected to a pointP between the serially connected resistances and 8.5, and the cathode ofdevice $53 is connected to the negative terminal of a source 9i ofbiasing potential, the positive terminal of which is connected to groundthrough a coupling resistance 82. A suitable condenser 59 is connectedin shunt to the serially connected source and resistance 9?:for-bypassing high frequency currents around such serially connectedcircuit elements.

Source SI maintains the cathode of device 98 negative with respect toground and also with respect to the control electrode 122, so that inthe absence of asignal voltage current tends to flow from source 9|through resistance 92 to ground, then through resistance 82, through theresistance 95, to the control electrode E22, and through the cathode ofdevice 90 and back to the negative terminal of source 9|. Such currentflow maintains continuously a bias potential across resistance 82 ofsuch polarity as to maintain control electrode I06 of device 8| negativewith respect to ground.

When the signal appearing across tuned circuit 79, 80 is of greater thana predetermined amplitude the conductivity of device 96 changes not onlyas the result of the increased voltage drop across resistance 82 butalso as the result of the increased voltage drop across resistance 92.Enhanced signal rectification between control electrode I66 and thecathode of the device BI produces a sufficiently large negative biasacross resistance 82 to cut off the anode current of device 90 so thatultimately the voltage drop across resistance 8 is only that produced bya relatively small s ace current flowing to device BI. A correspondinglyhigh direct current potential appears on anode H3 of the limitingamplifier device 8| and its transconductance is correspondingly greatlyincreased and the signals are efficiently transferred from the tunedcircuit '19, 86 to the tuned circuit 86, 81 with an effective limitingof the maximum amplitude of both the positive and negative half cyclesof the signals.

With this arrangement, in which the limiter device SI is made effectiveor ineffective to pass signals from the tuned circuit '19, 86 to thetuned circuit 86, 81, as such signals in the tuned circuit I9, 86 areabove or below a predetermined intensity, the selectivity curves of thetuned circuits I6, 1! and I9, 80 and other tuned circuits in the radiofrequency amplifier stages 53 and I3, oscillator modulator 14 andintermediate frequency amplifier 75 are substantially fiat topped over afrequency range corresponding to the frequency range illustrated in l aslinear within the parts B and C of the curve 5. The selectivity curveoutside of the frequency range is such that a very sharply reducedtransfer of energy is produced through the above mentioned tunedcircuits outside of that frequency range, that is, in the regioncorresponding to the back slopes E and I of the curve 5 in Fig. 1 andwithin the frequency ranges marked A and D. Accordingly, if those tunedcircuits are so adjusted in band width that an incoming signal lieswithin the range A or within the range D shown in Fig. 1 such signal issufiiciently attenuated at the tuned circuit I9, 86 of Fig. 2 so as tobe below the predetermined intensity at which the discharge device 8Itransfers signals to the tuned circuit 86, 81. It is only when a signalon the antenna I lies within the linear frequency range of the curve 5of Fig. 1, that is, within the frequency ranges B and C of Fig. 1, thatit appears in the tuned circuit I9, 86 in sufficient intensity to biasdevice 90 beyond cutoff, with the result that the signal is amplified bydevice 8|.

As has been mentioned previously, a portion of the audio signalamplified by discharge device I62 is fed to rectifying discharge deviceI3I through condenser III. The cathode of device I3I is grounded througha tapped resistance I36 which is shunted by an audio frequency bypasscondenser I3'I. Rectified voltage appearing between the tap onresistance I36 and ground is filtered by serially connected filterinductance II! and shunt connected filter condenser II4. A portion ofthe voltage appearing across resistance I36 is filtered and appliedacross resistance 92 when a serially connected switch H6 is closed byconnecting the movable tap on resistance I36 to one terminal ofresistance 92, the other terminal of resistance 92 and one terminal ofresistance I36 being grounded. This rectified voltage from device I3Icauses the ungrounded terminal of resistance 92 to become more positiveas the signal voltage applied to discriminator 9'! increases, and thenet continuous negative bias voltage appearing on grid I22 of devicebecomes sumcient to cut off current flow in device 90 with the resultthat the anode voltage on device BI, and consequently thetransconductance of that device, increases. The output signal fromdevice BI correspondingly increases with a resulting increase in theamplitude of the audible signal from the reproducer I69.

In Figure 2, when switch 10 is connected as shown to the groundedterminal. the deviation sensitivity of the receiver is controlled inaccordance with the audio voltage output. When switch III; is closed andwhen switch it is connected to filter inductance II5, the deviationsensitivity of the receiver is controlled not only by the audio voltageoutput but also by the signals appearing across the output ofintermediate frequency transformer 18.

The receiver shown herein may be tuned by conventional means, forexample, the tuning means may comprise variable condensers or variableinductances. Such tuning means are represented by the variablecapacitance 62A and 66 shown in Fig. 2. The grid input circuit and theplate output circuit of the radio frequency amplifier 13 may also betuned. Tuning may be connected in conventional fashion.

It is preferable, but not essential, to use a pentode type of dischargedevice in the main signal translating channel. While the auxiliarycontrol discharge device 90 is shown as a triode type, it may bedesirable, under certain conditions, to use a tetrode or pentode inplace of the triode.

While the particular embodiments of the present invention have beenshown and described. it will be obvious to those skilled in the art thatchanges and modifications may be made without departing from thisinvention in its broader as pects, and therefore the aim in the appendedclaims is to cover all such changes and modifications as fall within thetrue spirit and scope of this invention.

I claim:

1. A frequency deviation responsive device including a source offrequency modulated carrier waves of varying amplitudes and various meanfrequencies, means for selectively amplifying a carrier wave from saidsource, a frequency modulation signal detector of the resonant typehaving a principal linear frequency response region and adjacentnon-linear frequency response regions, said amplifying means having afrequency sclectivity characteristic with substantially more attenuationin said non-linear regions than in said principal linear region, anelectron discharge device having a control electrode, anode and cathode.an input circuit connected between said control electrode and cathodeincluding means for producing self-bias voltage in response to a carrierwave impressed on said control electrode, means for impressing aselected carrier wave from said amplifying means on said input circuit,an output circuit connected between said anode and cathode and coupledto said detector, said device and circuits being arranged to limit to apredetermined maximum amplitude any carrier wave impressed thereon, asecond electron discharge device having a control electrode, anode, andcathode, means for impressing self-bias voltage from the controlelectrode of said first discharge device on the control electrode ofsaid second device negatively with respect to the oathode of said firstdevice, and means for connecting the anode and cathode of said seconddevice to said output circuit effectively in shunt with the anode andcathode of said first device, said second device having sufficienttransconductance with said circuits adjusted for satisfactory signaldetection in said principal linear region that substantially no signaldetection takes place in said adjacent regions.

2. A frequency deviation responsive device including a source offrequency modulated carrier Waves of varying amplitudes and various meanfrequencies, means for selectively amplifying a carrier wave from saidsource, a frequency modulation signal detector of the resonant typehaving a principal linear frequency response region and adjacentnon-linear frequency response regions, said amplifying means having afrequency selectivity characteristic with substantially more attenuationin said non-linear regions than in said principal linear region, anelectron discharge device having a control electrode, anode and cathode,an input circuit connected between said control electrode and cathodeincluding means for producing self-bias voltage in response to a carrierwave impressed on said control electrode, means for impressing aselected carrier wave from said amplifying means on said input circuit,an output circuit connected between said anode and cathode and coupledto said detector, said device and circuits being arranged to limit to apredetermined maximum amplitude any carrier wave impressed thereon, asecond electron discharge device having a, control electrode, anode, andcathode, means for impressing self-bias voltage from the controlelectrode of said first discharge device on the control electrode ofsaid second discharge device and for biasing the cathode of said seconddevice negatively with respect to the cathode of said first device,means for connecting the anode and cathode of said second device to saidoutput circuit effectively in shunt with the anode and cathode of saidfirst device, said second device having sufiicient transconductance withsaid circuits adjusted for satisfactory signal detection in saidprincipal linear region that substantially no signal detection takesplace in said adjacent regions, means responsive to detected signals forproducing a bias voltage, and means for changing the conductivity ofsaid second device in joint response to said last mentioned bias voltageand said self-bias voltage.

3. A frequency deviation responsive device including a source offrequency modulated carrier waves of varying amplitudes and various meanfrequencies, means for selectively amplifying a carrier wave from saidsource, a frequency modulated signal detector of the resonant typehaving a principal linear frequency response region and adacentnon-linear frequency response regions, said amplifying means having afrequency selectivity characteristic with substantially more attenuationin said non-linear regions than in said principal linear region, anelectron discharge device having a control electrode, anode and cathode,an input circuit connected between said control electrode and cathodeincluding means for producing self-bias voltage in response to a carrierwave impressed on said control electrode, means for impressing aselected carrier wave from said amplifying means on said input circuit,an output circuit connected between said anode and cathode and coupledto said detector, said device and circuits being arranged to limit to apredetermined maximum amplitude any carrier wave impressed thereon, asecond electron discharge device having a control electrode, anode, andcathode, means for impressing self-bias voltage from the controlelectrode of said first discharge device on the control electrode ofsaid second discharge device and for biasing the cathode of said seconddevice negatively with respect to the cathode of said first device,means for connecting the anode and cathode of said second device to saidoutput circuit effectively in shunt with the anode and cathode of saidfirst device, said second device having suflicient transconductance withsaid circuits adjusted for satisfactory signal detection in saidprincipal linear region that substantially no signal detection takesplace in said adjacent regions, means responsive to detected signals forproducing a bias voltage and means responsive to said last mentionedbias voltage for aiding the change in conductivity of said second deviceproduced by said self-bias voltage.

ROBERT T. THOMPSON.

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